Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery has a positive electrode including an active material of complex oxides capable of storing and emitting lithium ions, a negative electrode, a separator, and an electrolytic solution made of a nonaqueous solvent. A discharge curve of this battery when being discharged with a constant power has two or more points of step-like flections near the end of electrical discharge in a range of 5% to 20% of a discharge capacity thereof as determined from an initial discharge voltage in a state of full charge to a discharge-end voltage.

RELATED APPLICATION

This application is a national phase of PCT/JP2006/300343 filed on Jan.13, 2006, which claims priority from Japanese Application No. JP2005-007401 filed on Jan. 14, 2005 and Japanese Application No. JP2005-377954 filed on Dec. 28, 2005 the disclosures of which Applicationsare incorporated by reference herein. The benefit of the filing andpriority dates of the International and Japanese Applications isrespectfully requested.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery capable of discharging a large current, with a high capacity andan excellent cycle characteristic.

BACKGROUND ART

In recent years, nonaqueous electrolyte secondary batteries are oftenused as main power sources of mobile apparatuses such as mobiletelecommunications devices and portable electronic devices since theycan provide high energy density at high voltage. There are also demandsrecently for nonaqueous electrolyte secondary batteries of light-weightand small size, yet capable of delivering large discharge currentsbecause of the needs for installing them in automobiles, and for usewith direct current-driven heavy tools.

Despite of these demands, nonaqueous electrolyte secondary batteriesusually result in temperature rises due to increase in heat generated bythe Joule effect when large currents are discharged since the batterieshave internal direct current resistances. Generally, nonaqueous solventin an electrolytic solution used for the nonaqueous electrolytesecondary batteries contains a component that boils or resolves when thetemperature exceeds about 90° C. For this reason, charge and dischargecapacity decreases drastically when repeating such a cycle that causesthe battery temperature to exceed 90° C. during charging anddischarging. Numerous studies are being made in efforts to solve such aproblem.

An internal resistance of any battery is divided into a reactiveresistance related to reaction of the battery, a resistance attributableto the electrolytic solution and separator, and a resistance of currentcollectors. In order to lower the resistance of current collectors amongthese resistances, there is a work disclosed in Japanese PatentUnexamined Publication, No. H11-233148, for example, which decreases thedirect current resistance of battery by improving a structure ofconnections of a positive electrode and a negative electrode to exteriorparts. This work is aimed at reducing the Joule heat generated insidethe battery. An electric power (i.e., output power) is the product of acurrent and a voltage. Therefore, in the case of an apparatus requiringa high power by way of constant power discharge, such as a power tool,there occurs a rise in the discharge rate (i.e., discharge current) whenthe voltage decreases quickly near the end of electric discharge. Sincethe decrease in voltage is attributed to a material of the positiveelectrode, the above technique of decreasing the direct currentresistance does not provide a direct effect in this case.

There is also an idea of installing a temperature sensor on a surface ofthe battery, and using a control to stop operation of an apparatus whena surface temperature of the battery reaches a predetermined value orhigher, as disclosed in Japanese Patent Unexamined Publication, No.2004-179085, for example. Besides the electrolytic solution, however,nonaqueous electrolyte secondary batteries contain other materials thatgenerate heat under high temperatures, such as an active material forpositive electrode in the end of electric discharge. In other words, theactive material for the positive electrode generates a large amount ofheat by reaction if it is discharged to a low voltage potential. Thereis thus a possibility that the battery becomes overheated if thepredetermined control temperature is set too high in the above control.On the other hand, the discharge capacity decreases drastically if thepredetermined control temperature is set too low.

In addition, there is another idea of deterring the drastic decrease involtage near the end of electrical discharge by using two kinds ofactive materials having different ranges of average discharge voltagefor the positive electrode as disclosed, for example, in Japanese PatentUnexamined Publication, No. H09-180718. In an example that uses suchactive materials for the positive electrode, there gives rise to aproblem described hereafter when a large current is discharged as in thecase of power tools. That is, the battery comes to a discharge-endvoltage due to a rise in electrical potential of the negative electrodein reality at the end of electrical discharge, even though it isintended to bring the battery voltage to the discharge-end voltage bydecreasing an electrical potential of the positive electrode. Thisimpedes the effect of voltage control by means of the discharge voltagein the positive electrode, and thereby it makes the battery in the stateof overheating near the end of electric discharge. As a result, it makesa reduction of designed capacity since it becomes necessary to increasean irreversible capacity of the positive electrode larger than that ofthe negative electrode in order to avoid this problem.

SUMMARY OF THE INVENTION

The present invention is directed to address the above problems, and itis an object of this invention to provide a nonaqueous electrolytesecondary battery capable of delivering a large capacity while avoidinggeneration of heat substantially in the end of electrical dischargeespecially in such application as power tool that requires a largedischarge current. The nonaqueous electrolyte secondary battery of thisinvention has a positive electrode, a negative electrode, a separatordisposed between the positive electrode and the negative electrode, andan electrolytic solution containing a nonaqueous solvent. The positiveelectrode includes complex oxides capable of storing and emittinglithium ions, as an active material. The negative electrode can alsostore and emit lithium ions. A discharge curve of this battery whenbeing discharged with a constant power has two or more points ofstep-like flection near the end of electrical discharge in a range of 5%to 20% of its discharge capacity as determined from an initial dischargevoltage in a state of full charge to a discharge-end voltage. Thestructure composed in this manner can moderate a rate of voltagedecrease as well as a rate of current increase in the end of discharge,thereby providing the nonaqueous electrolyte secondary battery with anadvantage of reducing a steep rise in temperature of the battery whenbeing discharged by drawing a large current. In addition, the nonaqueouselectrolyte secondary battery of the present invention uses a positiveelectrode containing a mixture of at least two kinds of lithium-basedcomplex oxides having different average discharge voltages as activematerials. One of the active materials having the lowest averagedischarge voltage is so mixed that its amount in capacity comes to 5% ormore but not more than 20% of that of the total amount of the activematerials. The structure composed in this manner achieves a dischargecurve having a point of step-like flection, so as to moderate the rateof voltage decrease as well as the rate of current increase in the endof discharge, thereby providing the nonaqueous electrolyte secondarybattery with the advantage of reducing the steep rise in temperature ofthe battery when being discharged with a large current.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a nonaqueous electrolyte secondary batteryaccording to an exemplary embodiment of the present invention; and

FIG. 2 is a graphical representation showing changes in dischargevoltages and temperatures of an embodied example and a comparisonexample as they are being discharged.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view of a cylindrical battery representing oneexample of a nonaqueous electrolyte secondary battery according to anexemplary embodiment of the present invention. In this battery, anelectrode block having negative electrode 1, positive electrode 2, andseparator 3 disposed between these electrodes to prevent them frombecoming a direct contact, which are then wound together and placedinside battery case 4. Negative electrode 1, positive electrode 2 andseparator 3 are impregnated with an electrolytic solution (not show)containing a nonaqueous solvent. An opening of battery case 4 is closedwith sealing plate 5, to hence complete the structure of this sealedtype battery.

Negative electrode 1 has collector 1A, mixture layer 1B containing anegative electrode active material and disposed over collector 1A, andlead wire 1C connected to collector 1A. A carbon material, a crystallineor non-crystalline metal oxide, and the like material are capable ofstoring and emitting lithium, and therefore used as the active materialscontained in mixture layer 1B. The carbon material may be any ofingraphitizable carbon materials such as coke and glassy carbon, and anyof graphite made of a highly crystallized carbon material having a growncrystalline structure, to name a few. More specifically, the usablematerial includes any of pyrolytic carbons, cokes (e.g., pitch coke,needle coke, petroleum coke, and the like), graphite, glassy carbons,burned products of organic polymer compounds (e.g., a phenol resin, afuran resin, etc. which are burned and carbonized at a propertemperature), carbon fibers, activated carbons, and so on. It isdesirable to use a material having the small irreversible capacity amongthem because that is the one meeting the spirit of this invention.

Mixture layer 1B is obtained from a paste formed by mixing the negativeelectrode active material, a binder and a solvent, which is coated oncollector 1A, and dried. An electrically conductive material such ascarbon black may be added to the paste as needed. Mixture layer 1B alsobe roll-pressed after it is dried. The binder can be any of the ordinarybinding agents known to be used for this kind of batteries. Specificexamples include polyethylene, polypropylene, poly-tetrafluoroethylene,poly-vinylidene fluoride, styrene butadiene rubber, and the like.Collector 1A and lead wire 1C can be composed of a metal such as copperand nickel.

Positive electrode 2 has collector 2A, mixture layer 2B containing apositive electrode active material and disposed over collector 2A, andlead wire 2C connected to collector 2A. Mixture layer 2B is obtainedfrom a paste made by mixing the positive electrode active material, abinder and a solvent, which is coated on collector 2A, and dried. Anelectrically conductive material such as carbon black and graphite maybe added to the paste as needed. Mixture layer 2B may also beroll-pressed after it is dried. The binder can be the same material asis used for negative electrode 1. Collector 2A and lead wire 2C may becomposed of a metal such as aluminum, stainless steel, and titanium.

The electrolytic solution can be obtained by dissolving supporting saltinto nonaqueous solvent. For the nonaqueous solvent, a solvent having acomparatively high dielectric constant and not easily resolvable by thegraphite composing negative electrode 2, such as ethylene carbonate(hereinafter referred to as “EC”), ethyl-methyl carbonate (referred toas “EMC”), dimethyl carbonate (referred to as “DMC”), and the likematerials are used as a primary solvent. It is desirable to use EC asthe primary solvent especially when negative electrode 2 is made of agraphite material. However, it is also possible to use a compound of ECin which a halogen element is substituted for a hydrogen atom.Alternatively, although propylene carbonate (referred to as “PC”) isreactive to graphite material, it can provide a better characteristicwhen a part of it is replaced with a second element solvent, as comparedto the primary solvent of EC or a compound of EC of which the halogenelement is substituted for the hydrogen atom. In addition, it is alsodesirable to use the nonaqueous solvent together with another solvent oflow viscosity, which increases the conductivity to improve a currentcharacteristic, and reduces its reactivity to a lithium metal to therebyimprove safety.

On the other hand, the supporting salt can be of any type of lithiumsalt without limitations so long as it is dissolvable in the nonaqueoussolvent and shows an ionic conductivity. For example, LiPF₆, LiClO₄,LiAsF₆, LiBF₄, LiB(C₆H₅)₄, LiCH₃SO₃, CF₃SO₃Li, LiCl,LiN(CnC_(2n+1)SO₂)₂, LiBr, etc. can be used. It is especially desirableto use LiPF₆ as the supporting salt. Any one of these supporting saltsmay be used singly, or two or more of them may be used as a mixture.

A material usable for battery case 4 can be iron, nickel, stainlesssteel, aluminum, titanium, and the like. Battery case 4 may be treatedwith plating or the like to prevent electrochemical corrosion by thenonaqueous electrolyte during electric charges and discharges of thebattery.

In a process of manufacturing the nonaqueous electrolyte secondarybattery constructed as above, negative electrode 1 and positiveelectrode 2, both having a ribbon-like shape as described above are puttogether with separator 3 made of a micro-porous polyethylene film, forexample, placed between them. They are then rolled along theirlongitudinal direction in many times to form an electrode block of woundtype. This electrode block is placed inside battery case 4 made of iron,of which an interior is nickel plated, and electrically insulating plate6 being inserted in the bottom thereof. One end of lead wire 1C iswelded to battery case 4 to collect an electric current from negativeelectrode 1. This makes battery case 4 in electrical continuity withnegative electrode 1 to provide a negative terminal of the nonaqueouselectrolyte secondary battery. Furthermore, one end of lead wire 2C iselectrically connected with sealing plate 5 through current-break sheet5B to collect an electric current from positive electrode 2.Current-break sheet 5B has a function to break the current in responseto an internal pressure of the battery. This makes sealing plate 5 inelectrical continuity with positive electrode 2 to provide a positiveterminal of the nonaqueous electrolyte secondary battery.

After the inner space of battery case 4 is filled with the electrolyticsolution prepared by dissolving the supporting salt in the nonaqueoussolvent, sealing plate 5 is inserted in the opening of battery case 4.Battery case 4 is then crimped over gasket 5A made of an insulationresin coated with a sealan, and to complete the cylindrical nonaqueouselectrolyte secondary battery on which sealing plate 5 is secured.

It is desirable for this nonaqueous electrolyte secondary battery to beequipped with current-break sheet 5B, which acts as a safety valve torelease gases from the inside if a pressure builds up beyond apredetermined value as described.

Description is provided next of the positive electrode active materialcontained in mixture layer 2B. A discharge curve of this nonaqueouselectrolyte secondary battery when being discharged with a constantpower has two or more points of step-like flection near the end ofelectrical discharge in a range of 5% to 20% of its discharge capacityas determined from an initial discharge voltage in the state of fullcharge to a discharge-end voltage. These points of step-like flectionhere mean points that indicate boundaries between two phases where adischarge mechanism changes from one to the other, or points where aninclination of voltage drop becomes steep near the end of electricdischarge.

Such a discharge curve can be achieved by composing the positiveelectrode active material as described hereinafter. The positiveelectrode active material is obtained by mixing at least two kinds oflithium-based complex oxides having different average dischargevoltages. That is, positive electrode 2 has the positive electrodeactive material which includes at least a first active material oflithium-based complex oxide and a second active material of anotherlithium-based complex oxide having a lower average discharge voltagethan that of the first active material. The second active material is soadded that its amount in capacity comes to 5% or more but not more than20% of a total amount of capacity of the active materials. The structurecomposed in this manner thus achieves the discharge curve having pointsof the step-like flection. Such a discharge characteristic gives amoderate rate of voltage decrease near the end of electric discharge. Asa result, it moderates a rate of current increase near the end ofelectric discharge, and reduces a steep rise in temperature of thebattery when being discharged with a large current. In the nonaqueouselectrolyte secondary battery composed as described above, since theinternal temperature can be checked more accurately with a temperaturesensor, it helps ease control of the electric charges and discharges,and thereby prolongs a usable life of the battery.

It is conceivable that combinations of such active materials include acombination of Li_(0.98)CoO₂ and LiMnO₂, another combination ofLi_(0.98)CoO₂ and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, and the likecombinations. In this instance, a combination of a composite “A”expressed as Li_(x)MO₂ (where “M” denotes a 3d transition metal) as thefirst active material, and another expressed as LiMnO₂, having anaverage discharge voltage of 2 to 3V as the second active material canmake the decrease in voltage most gently near the end of electricdischarge, thereby alleviating most the drastic rise in temperature ofthe battery.

It is further desirable that the composite “A” expressed as Li_(x)MO₂(where “M” denotes a 3d transition metal) satisfies 0.9≦x≦0.98. Adesigned capacity of the battery decreases substantially in proportionto a decrease in the theoretical capacity of the positive electrode ifthe x value is less than 0.9. If the x value exceeds 0.98, on thecontrary, an irreversible capacity of the positive electrode decreases,and the battery comes to its discharge-end voltage due to a rise involtage potential of the negative electrode. This impairs the effect ofvoltage control using the discharge voltage of the positive electrode,thereby causing the battery to heat up at the end of electric discharge.

It is also desirable that a ratio of LiMnO₂ by weight is at least 2% andat most 20% of the total amount of the active materials. If the ratio byweight of LiMnO₂ is less than 2%, it does not provide the effect ofmoderating the drastic decrease of voltage near the discharge-endvoltage of 3.0 to 2.5V, and it consequently leads the battery to heat upat the end of discharge. If this ratio by weight of LiMnO₂ exceeds 20%,on the contrary, it does not provide a sufficient energy density sincethe average discharge voltage decreases substantially due to the excessamount of LiMnO₂. Moreover, it increases the discharge current as theaverage discharge voltage decreases when the battery is being dischargedwith a constant power. This increases generation of the Joule heat,which heats up the battery. This desirable range of adding amounts isalso applicable to other materials besides LiMnO₂ when used as theactive materials having the lowest value of average discharge voltage.

Furthermore, it is desirable that one of composites “A” is any ofmaterials expressed as Li_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂ (where 0.9≦x≦0.98,0.3≦y≦0.4, and 0.3≦z≦0.4) and Li_(x)Ni_(y)Co_(z)Al_(1−y−z)O₂ (where0.9≦x≦0.98, 0.55≦y≦0.8, and 0.15≦z≦0.3). Or, these materials may becombined together. Use of the above materials for composites “A” canfurther moderate the decreasing rate of discharge voltage near the endof electric discharge, and reduce the heat attributable to the drasticincrease of discharge current.

The nonaqueous electrolyte battery of this invention is not limited tononaqueous electrolyte secondary batteries of the type that usesnonaqueous electrolytic solution as the electrolyte, as described above,but it is also adaptable for use with an electrolyte of gelatinous type.In addition, the nonaqueous electrolyte secondary battery of thisinvention has no specific limitation in its shape, and it can be of anysuch shape as cylindrical, square, coin type, button type, and the like.It can also be fabricated into any size including thin and largedimensions.

In the nonaqueous electrolyte secondary battery according to thisexemplary embodiment of the invention, description is now provided indetail of certain embodied examples, in which changes are made incompositions of active materials used for the positive electrodes

Embodied Example 1

A positive electrode active material prepared here included a 90-weightportion of composite “A” in a powder form and a 10-weight portion ofcomposite “B” also in a powder form as described hereinafter. A materialused for composite “A” was Li_(0.95)CoO₂ obtained from Li₂CO₃ and Co₃O₄which were mixed and burned for 10 hours at 900° C. A material used forcomposite “B” was Li_(0.95)Ni_(0.35)Mn_(0.35)Co_(0.3)O₂ obtained fromLi₂CO₃, Co₃O₄, NiO and MnO₂, which were mixed and burned for 10 hours at900° C. A total of 100-weight portion of these active materials, a2.5-weight portion of acetylene black as an electrically conductivematerial, and a 4-weight portion of poly-vinylidene fluoride as a binderwere mixed together. This mixture was suspended in an aqueous solutionof carboxymethyl-cellulose, and made into a paste form. This paste wascoated on both surfaces of collector 2A made of an aluminum foil having0.03 mm in thickness, and after having been dried it was roll-pressed tocomplete positive electrode 2 measuring 0.1 mm in thickness, 52 mm inwidth and 1,800 mm in length.

A material used as a negative electrode active material was made ofmesophase spheres which were graphitized at as high a temperature as2,800° C. (hereinafter referred to as mesophase graphite). This materialwas suspended in a styrene butadiene rubber emulsion having 1 wt-% ofsolid content and a hydroxyl-methyl cellulose solution also having 1wt-% of solid content, and made into a paste form. This paste was coatedon both surfaces of a copper foil having 0.02 mm in thickness, and afterhaving been dried it was roll-pressed to complete negative electrode 1measuring 0.1 mm in thickness, 57 mm in width and 1,860 mm in length.

Lead wire 2C made of aluminum and lead wire 1C made of copper wereattached to positive electrode 2 and negative electrode 1 respectively.An electrode block was constructed thereafter by winding round them intoa spirally-wound form together with separator 3 made of a polyethyleneof 0.025 mm in thickness, 60 mm in width and 4,000 mm in length, placedtherebetween. This block was inserted in battery case 4 of 26.0 mm indiameter and 65 mm in height. An electrolytic solution used was asolution containing LiPF₆ dissolved to a concentration of 1.25 mol/dm³in a solvent consisting of EC, EMC and DMC mixed to a ratio of 10:10:80by volume. Battery case 4 filled with this electrolytic solution wasprovided with a temperature sensor mounted in the center area of thebattery, and the opening was sealed to complete the battery with anominal capacity of 2.5 Ah. This battery was identified as embodiedexample 1.

Embodied Example 2

A positive electrode active material prepared here included a 90-weightportion of composite “A” in a powder form and a 10-weight portion ofcomposite “B” also in a powder form as described hereinafter. A materialused for composite “A” was Li_(0.95)CoO₂ obtained from Li₂CO₃ and Co₃O₄which were mixed and burned for 10 hours at 900° C. A material used forcomposite “B” was Li_(0.95)Ni_(0.55)Co_(0.30)Al_(0.15)O₂ obtained fromLi₂CO₃, Co₃O₄, NiO and Al₃O₄, which were mixed and burned for 10 hoursat 900° C. A battery similar to embodied example 1 was made with onlydifferences as set forth above. This battery is identified as embodiedexample 2.

Embodied Example 3

A positive electrode active material prepared here comprises a 90-weightportion of composite “A” in a powder form and a 10-weight portion ofcomposite “B” also in a powder form as described hereinafter. A materialused for composite “A” w as Li_(0.95)CoO₂ obtained from Li₂CO₃ and Co₃O₄which were mixed and burned for 10 hours at 900° C. LiMnO₂ was used forcomposite “B”. LiMnO₂, was obtained from lithium hydroxide (LiOH.H₂O)and manganite (γ-MnOOH) which were left for 3 hours in an air ambientmaintained at a temperature of 99° C. or above and a humidity saturatedto 17.05 (in kg-vapor/kg-dry air) or higher. A battery similar toembodied example 1 was made with only differences as set forth above.This battery was identified as embodied example 3.

Embodied Examples 4 to 6

Batteries similar to embodied example 3 were made except that thematerial of composite “A” of the embodied example 3 was replaced byLi_(0.85)CoO₂, Li_(0.90)CoO₂ and Li_(0.98)CoO₂ for respective examples.These batteries were identified as embodied examples 4 to 6.

Embodied Examples 7 to 9

Batteries similar to embodied example 3 were made except that the ratioby weight of composite “A” portion vs. LiMnO₂ portion in the embodiedexample 3 was changed to 98:2, 95:5, and 80:20 for respective examples.These batteries were identified as embodied examples 7 to 9.

Embodied Examples 10 and 11

The material used as composite “A” in the embodied example 3 is replacedby Li_(0.95)Ni_(0.35)Mn_(0.35)Co_(0.3)O₂ andLi_(0.95)Ni_(0.40)Mn_(0.40)Co_(0.20)O₂ for respective examples. Thesematerials were obtained by mixing Li₂CO₃, Co₃O₄, NiO and MnO₂ in avariety of mixing ratios, and burning for 10 hours at 900° C. Batteriessimilar to embodied example 3 except for the above were made, and thesebatteries were named embodied examples 10 and 11.

Embodied Examples 12 and 13

The material used as composite “A” in the embodied example 3 is replacedby Li_(0.95)Ni_(0.80)Co_(0.15)Al_(0.05)O₂ andLi_(0.95)Ni_(0.55)Co_(0.30)Al_(0.15)O₂ for respective examples. Thesematerials were obtained by mixing Li₂CO₃, Co₃O₄, NiO and Al₃O₄ in avariety of mixing ratios, and burning for 10 hours at 900° C. Batteriessimilar to embodied example 3 except for the above were made, and thesebatteries were named embodied examples 12 and 13.

Comparison Example 1

A battery similar to embodied example 3 was made using onlyLi_(0.95)CoO₂ as a positive electrode active material, like composite“A” in the embodied example 3, and this battery was named comparisonexample 1.

Comparison Example 2

A battery similar to embodied example 3 was made using only LiMnO₂ as apositive electrode active material as in the case of embodied example 3,and this battery was named comparison example 2.

Comparison Example 3

A battery similar to embodied example 3 was made using LiCoO₂ ascomposite “A” as in the case of embodied example 3, and this battery wasnamed comparison example 3.

Table 1 shows specifications of the individual batteries discussed aboveand results of evaluations conducted in the following manner. Table 1also shows results of evaluations made on characteristics of thebatteries.

TABLE 1 Voltage at 100 W Capacity Flection Amount of Discharge HighestRetention Point in Compound of Compound of Composite Capacity Temp.Ratio Discharge Composite A Composite B B (wt-%) (%) (° C.) (%) (V)Example 1 Li_(0.95)CoO₂ Li_(0.95)Ni_(0.35)Mn_(0.35)Co_(0.30)O₂ 10 95 7565 3.6 Example 2 Li_(0.95)CoO₂ Li_(0.95)Ni_(0.55)Co_(0.30)Al_(0.15)O₂ 1095 73 67 3.3 Example 3 Li_(0.95)CoO₂ LiMnO₂ 10 95 70 70 3.0 Example 4Li_(0.85)CoO₂ LiMnO₂ 10 75 70 70 3.0 Example 5 Li_(0.90)CoO₂ LiMnO₂ 1095 70 70 3.0 Example 6 Li_(0.98)CoO₂ LiMnO₂ 10 95 70 70 3.0 Example 7Li_(0.95)CoO₂ LiMnO₂ 2 96 74 65 3.0 Example 8 Li_(0.95)CoO₂ LiMnO₂ 5 9672 65 3.0 Example 9 Li_(0.95)CoO₂ LiMnO₂ 20 92 67 75 3.0 Example 10Li_(0.95)Ni_(0.35)Mn_(0.35)Co_(0.30)O₂ LiMnO₂ 10 95 65 75 3.0 Example 11Li_(0.95)Ni_(0.40)Mn_(0.40)Co_(0.20)O₂ LiMnO₂ 10 95 65 75 3.0 Example 12Li_(0.95)Ni_(0.80)Co_(0.15)Al_(0.05)O₂ LiMnO₂ 10 95 65 75 3.0 Example 13Li_(0.95)Ni_(0.55)Co_(0.30)Al_(0.15)O₂ LiMnO₂ 10 95 65 75 3.0 ComparisonLi_(0.95)CoO₂ — 0 95 90 10 — Example 1 Comparison — LiMnO₂ 100 20 35 100— Example 2 Comparison LiCoO₂ LiMnO₂ 10 95 90 10 — Example 3

First, evaluations were made for constant power dischargecharacteristics with a high load. In an ambient temperature of 20° C.,the batteries w ere charged up to 4.2V with a constant current of 2.6 A,and they were further charged with the constant voltage up to the endcurrent of 0.26 A. After an interval of 20 minutes, the batteries weredischarged by drawing a constant current of 0.52 A until their voltagesreached 2.0V. This was noted as a first cycle. The batteries wereconsecutively charged under the same conditions as the first cycle, andafter an interval of 20 minutes they were again discharged for aconstant output power of 100 W until their voltages reached 2.0V. Thiswas noted as a second cycle. During these processes, the batteries wereexamined for their ratios of discharge capacities in the second cyclewith respect to those of the first cycle, as well as their highesttemperatures recorded after the end of the second cycle. Examinationswere also made at the same time for flection voltages in their dischargevoltage curves during discharges of the first cycle. FIG. 2 showschanges in the discharge voltages and the temperatures in the course ofelectrical discharge of the embodied example 1 and comparison example 1,just for examples.

Characteristics of the charge and discharge cycle were examined next. Inthe temperature ambient of 20° C., a charge and discharge cycles wererepeated under the same condition as those of the second cycle, whichwas for the evaluation of a high-rate discharge characteristic. Ratiosof discharge capacities in the 300th cycle with respect to those of thefirst cycle were recorded as capacity retention ratio.

As shown in Table 1, the batteries represented as the embodied examples1 and 2 exhibited longer life and less heats at the end of discharges ascompared to those of the comparison example 1, as they used composite“A” of Li_(0.95)CoO₂ having a high average discharge voltage, added witheither one of the composite “B”, Li_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂ andLi_(x)Ni_(y)CO_(z)Al_(1−y−z)O₂ having a low average discharge voltage.They also exhibited larger discharge capacities as compared withcomparison example 2.

The discharge curve of comparison example 1 has only one point of thestep-like flection 11, as shown in FIG. 2, near the end of theelectrical discharge in a range of 5% to 20% of its discharge capacityas given from the initial discharge voltage in the state of full chargeto a discharge-end voltage. Flection point 11 occurs because of thesteep decline in voltage near the end of electrical discharge ofLi_(0.95)CoO₂. On the other hand, the discharge curve of embodiedexample 1 has three points of the step-like flection near the end ofelectrical discharge in a range of 5% to 20% of its discharge capacityas given from the initial discharge voltage in the state of full chargeto the discharge-end voltage. The first flection point 10A occursbecause of the steep decline in the voltage drop when the electricaldischarge of Li_(0.95)CoO₂ became close to its end. The second flectionpoint 10B shows that the discharge mechanism shifted from electricaldischarge of Li_(0.95)CoO₂ to electrical discharge ofLi_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂. And, the third flection point 10C occursbecause of the steep decline in the voltage drop when the electricaldischarge of Li_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂ became close to its end.There are cases in which the third flection point 10C does not occurdepending on a relation among a value of the average discharge voltageof the active material having a low average discharge voltage, an amountof the added active material, and the discharge-end voltage. In thismanner, it is desirable that the discharge curve has at least two pointsof the step-like flection near the end of electrical discharge in arange of 5% to 20% of the discharge capacity as given from the initialdischarge voltage in the state of full charge to the discharge-endvoltage. They can thus alleviate the temperature rise at the end ofelectrical discharge, as shown in FIG. 2.

In the embodied examples 3 through 13, any of the materials expressed asLi_(x)CoO₂ (where 0.9≦x≦0.98), Li_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂ (where0.9≦x≦0.98, 0.3≦y≦0.4, and 0.3≦z≦0.4), andLi_(x)Ni_(y)Co_(z)Al_(1−y−z)O₂ (where 0.9≦x≦0.98, 0.55≦y≦0.8, and0.15≦z≦0.3) were used as respective composite “A”. These composites “A”were added with LiMnO₂ having the average discharge voltage as low as2.5 to 3.0V of such an amount that it becomes 2 to 20% of the totalamount of the entire active materials by weight. These batteriesexhibited longer life and much less heats inside the batteries at theend of discharges as compared to those of the comparison example 1. Theyalso exhibited larger discharge capacities as compared with thecomparison example 2. Among those shown above, the embodied examples 10and 11, which used Li_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂ (where 0.9≦x≦0.98,0.3≦y≦0.4, and 0.3≦z≦0.4) and embodied examples 12 and 13, which usedLi_(x)Ni_(y)Co_(z)Al_(1−y−z)O₂ (where 0.9≦x≦0.98, 0.55≦y≦0.8, and0.15≦z≦0.3) as their respective composite “A” exhibited superiorperformance in both the discharge characteristic with high load outputand the charge/discharge cycle characteristic.

According to the nonaqueous electrolyte secondary battery in theexemplary embodiment of this invention, the decrease in voltage at theend of electrical discharge is alleviated, and the drastic temperaturerise inside the battery due to an increase of the discharge current isalleviated. It is so considered, however, that the comparison example 2is not suitable for such applications that require a large dischargecurrent because its discharge voltage is too low. The embodied example 4contained too low an amount of the element Li, which might have resultedin a small capacity due to an excessive irreversible capacity. It isthought that this has inevitably caused an increase in the dischargerate and a slight decrease in the discharge characteristic with theconstant output of high load. In the case of comparison example 3, thebattery capacity is determined by the battery voltage reaching thedischarge-end voltage due to a rise in the negative electrode potential,and it did not provide the effect of voltage control by means of thepotential change of positive electrode 2.

In the present exemplary embodiment, although what has been discussedabove is the cases wherein the batteries are discharged with a constantpower, the same advantage of alleviating the rise in heat at the end ofdischarge of composite “A” can be achieved even when the batteries aredischarged with a constant current since this invention provide theeffect of avoiding the composite “A” from being discharged to a lowvoltage potential.

INDUSTRIAL APPLICABILITY

The nonaqueous electrolyte secondary battery of the present invention issuitable for a power supply of a power tool and the like apparatus thatrequires a large discharge current and repeated cycle of charge anddischarge since it alleviates heat inside the battery at the end of highload discharge. Hence, it provides high utility in many industrialapplications.

1. A nonaqueous electrolyte secondary battery comprising: a positiveelectrode having an active material of a complex oxide capable ofstoring and emitting lithium ions; a negative electrode capable ofstoring and emitting lithium ions; a separator disposed between thepositive electrode and the negative electrode; and an electrolyticsolution containing a nonaqueous solvent, wherein discharge-end voltageof the nonaqueous electrolyte secondary battery is within 2.5V to 3.0V,the positive electrode contains a positive electrode active materialcomprising a first active material of lithium-based complex oxide and asecond active material of another lithium-based complex oxide having anaverage discharge voltage lower than an average discharge voltage of thefirst active material, an added amount of the second active material isat least 5% and at most 20% in capacity of a total amount of capacity ofthe positive electrode active material, and the first active material isa composite “A” expressed as Li_(x)MO₂, “M” denoting a 3d transitionmetal, x being given as 0.9≦x≦0.98, and the second active material isLiMnO₂ of which average discharge voltage is within 2V to 3V.
 2. Thenonaqueous electrolyte secondary battery according to claim 1, whereinthe composite “A” contains at least one of materials expressed asLi_(x)Ni_(y)Mn_(z)Co_(1−y−z)O₂, x, y, and z being given as 0.9≦x≦0.98,0.3≦y≦0.4, and 0.3≦z≦0.4, and Li_(x)Ni_(y)Co_(z)Al_(1−y−z)O₂, x, y, andz being given as 0.9≦x≦0.98, 0.55≦y≦0.8, and 0.15≦z≦0.3.